In the field of medicine, electronic endoscope systems are widely used. In a diagnostic procedure using the electronic endoscope system, an electronic endoscope is inserted into a body cavity of a patient to capture an image of an internal body part using a miniature image sensor provided at a tip of a slender insertion section. Some of the recent endoscope systems have an ability to switch the wavelength of illumination light for lighting the internal body part between several wavebands (see, United State Patent Application Publications No. 2008/0043469 A1 and No. 2006/0198620 A1). To the body tissue or structure difficult to observe under white visible light (hereinafter, normal light), for example, the light of a specific wavelength other than the normal light (hereinafter, special light, regardless of waveband) is emitted. Changing the illumination condition in this manner allows for emphasizing the body tissue, structure or lesion in the captured image.
Generally, the electronic endoscope system includes either a CMOS or CCD image sensor. The CMOS image sensor consumes less power, and fits on a single board along with peripheral circuitries. A typical CMOS image sensor incorporates a so-called rolling shutter technique that performs exposure of pixels and readout of signal charges (image signals) on every horizontal line in a single picture (frame). Therefore, with the rolling shutter technique, exposure timing is different on every horizontal line, and in capturing a moving object, the image is deformed in the moving direction. Moreover, the pixel amplifiers differ in performance, and may cause a particular noise to degrade the image quality.
The CCD image sensor, by contrast, can produce high-sensitivity, high-resolution images although it uses more power and has structure-inherent problems such as smear and blooming. Configured to expose all the pixels at once, the CCD image sensor can ensure the synchronization of the pixels in a single frame, preventing the deformation of a moving image. Since the organs are always moving, the CCD image sensor is often used in the endoscope systems.
Meanwhile, the recent technology improves the CMOS image sensors to achieve the image quality equal to or better than the CCD image sensors. This improvement gives an impetus for incorporating the mass-productive and low power consumption CMOS image sensors in the electronic endoscope systems.
As mentioned above, the CCD image sensors can synchronize all the pixels in a single frame. Accordingly, in capturing two or more successive frames (i.e., movie), changing the wavelength of the illumination light to the frame rate of the CCD image sensor will lead to produce successive images of different illumination conditions.
However, the CMOS sensors use the rolling shutter, which causes one frame difference in exposure timing between the first and last horizontal lines. Accordingly, if the wavelength of illumination light is changed at the frame rate in capturing two or more successive frames, the last horizontal line is exposed after the change of the wavelength. Each frame results to have the horizontal lines exposed under different illumination conditions, it is impossible to obtain the images of different illumination conditions successively.